In article ,
John B. wrote:

....

There seems to be three options. One, to use a ruler and measure from
pin to pin. Two to use a chain tool and measure from roller to roller.
Or three, to use some combination of the two.

Or perhaps there is a fourth - ignore the whole thing as a tempest in
a tea pot :-)

Chains are half an inch pitch. Put the chain under tension and
measure 24 links. At 12 and one eighth inches you're looking at 1%
elongation and you're likely to need to replace both the chain and
sprockets at the same time. At 12 and one sixteenth inches you're
at 0.5% elongation and you're likely to just need to replace the
chain.

Looks like I need to replace both the sprockets and chain on my
hybrid bike :-(

If your chainstays are short, you may need to measure just 20 links
and work with tenths and twentieths for the elongation.

Bear in mind that most riders will mainly use a few sprockets.
Those at the extreme end of the cassette get less use. You may find
the much used sprockets will not run well with a new chain even if
the old chain did not appear to have worn too much.
--
Dennis Davis